A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Commonly, transformers are used to increase or decrease voltages of alternating current in electric power applications.
Highly efficient, yet compact, power conversion is fundamental to continued profitable growth of the electrical industries including telecommunications and data processing. The end-use devices in said technological fields generally require electrical energy in a form, which is different from what is supplied by the electric grid. For example, telecommunication devices require input voltage of 50 volt DC and computers require 5 volt DC or 3 volt DC. In order to adapt the input voltage of the device to the proper level, transformers can be incorporated in the device. Additionally, transformers must be incorporated into the electronic devices for safety reasons, to galvanically isolate the end equipment from the 230 or 117 volt and 50 or 60 Hz AC of the grid.
Transformer size is directly related to switching frequency determined by a switching transistor, wherein higher frequencies allow using smaller volume of the transformer and result in higher efficiency, if the right design is chosen. Accordingly, in the past, transformer size was dictated by the relatively low operating frequencies offered by the existing switching transistors. Following development of MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistor) capable of converting power at frequencies well above 200 kHz, the major obstacles to ongoing performance improvements in power supplies are the limitations inherent in conventional “wire-wound-on-a-bobbin” transformers and companion inductors.
Transformers based on the planar principle eliminate virtually all the shortcomings of old-fashioned wire wound types. In a planar design, the windings are made of copper lead frames or printed circuit boards (flat copper spirals laminated into thin dielectric substrates). These windings are then sandwiched, along with appropriate insulators, between large area, yet thin, ferrite cores. Planar magnetics is probably the most cost effective solution for high frequency high power density power conversion equipment available today.
The principal advantage gained by the planar form is that a larger number of coils as a printed circuit and or lead frames can be fitted in to the equivalent space required by round-section wire. The planar printed coil opens up many design options, one of which is that the coil can be of any shape and width, and multiple coils on one face are possible. A wide conductor makes possible high current flow. Weight reduction is another benefit, this being of particular interest in aerospace applications. The planar circuits can be, and in most cases are interconnected with other circuits to generate a magnetic field and to meet a broad array of requirements.
One of the major hurdles of the planar transformer technology is the insulation of the copper lead frames or PCBs. The insulation of the conductive components of the planar transformers is typically performed by attaching an insulating polymeric film to the lead frame or using a soldering mask in case of the PCBs. Each lead frame or planar circuit usually needs to be insulated from the adjacent lead frame or circuit and almost always from the ferrite core passing through the planar coil. However, the terminals of the circuit need to be exposed so that electrical connections can be attached thereto. Thin Mylar®, Kapton® or high-temperature Nomex® films are generally used to cover the lead frames and provide the necessary interwinding insulation. Initially, the insulating films are cut into desired shape and then manually glued or hot pressed to the lead frame. Said procedure is prone to multiple problems, resulting from uneven shapes of the films, misaligned films from the opposite sides of the lead frame and as a result, insufficient insulation, or incomplete adhesion of the films. Being a non-automated process, application of the polymeric films to the lead frames requires trained manpower and large operation space, is time consuming and highly dependent on the alignment process precision. Additionally, relatively high thickness of the polymeric films, which can be handled by the workers, increases the overall thickness of the planar transformer. Furthermore, following the stacking of the isolated lead frames, the edges of the lead frames remain uncovered by the insulating material, which demands extra material for over-lapping and an additional step of the edge insulation is required.
For PCBs copper insulation comprising a liquid solidifying dielectric coating (e.g. solder mask) can be used. However, the thickness of the coating obtained shows significant variation, particularly in the vicinity of irregular copper shapes printed on the substrate, The coating can also become porous after drying, allowing an electrical discharge when the circuit is in use. In order to meet safety standards, such coatings require testing to conform to standards, and such testing increases production costs.
U.S. Pat. No. 6,882,260 to some of the inventors of the present invention is directed to a planar transformer circuit component comprising a flat lead frame coil or a first flat coil projecting from a first face of a printed circuit panel, said coil surrounding an aperture sized to allow projection therethrough of a ferrite core member, terminals for said coil being provided adjacent to an edge of said lead frame or panel, the exposed face and edges of said coil, including the edges of said aperture being insulated by a heat-resisting plastic film adhesively attached to said panel and to said coil face and to said coil edges, said film being provided with cut-outs leaving said terminals exposed for subsequent electrical connection.
One of the final steps of the preparation process of planar transformers' components includes cleaning of said parts in an aggressive cleaning solution, such as, for example, Vertrel® SFR, isopropyl alcohol (IPA) or Zestron® Co-150. Thus, a good adhesion between the insulating polymeric films and the conducting part is prerequisite for obtaining usable insulated conductive elements. Further, in order to meet the specific requirements of the planar transformers technology, the insulating layer of the conductive component should have an exceptionally high dielectric strength, such as at least about 1000 VAC per 1 Mil of an inch. Metal oxide coatings, which can be used, for example, as insulating films in element mounting boards of semiconductors, such as in US Patent Application No. 2014/0084452, are not suitable for insulating planar transformers' conductive elements, inter alia, due to the limitation of build-up thickness, metallurgical structure and cracking of the coating.
Electrophoretic deposition (EPD) is an electrochemical coating process, during which charged dispersed particles suspended in a liquid medium migrate under the influence of an electric field (electrophoresis) and are deposited onto an electrode. Any particles that can be used to form stable suspensions and that can carry a charge can be used in electrophoretic deposition. This includes materials such as polymers, pigments, dyes, ceramics and metals. There are several polymer types that have been used commercially for the EPD coatings, including epoxy and acrylic polymers.
The EPD process is useful for applying materials to any electrically conductive surface. Notable examples of industrial applications of EPD are formation of phosphors for cathode ray tubes and application of anti-corrosive primers for automotive body parts. Numerous advantages of the EPD process include uniform coating thickness, coating of complex fabricated objects, relatively high speed of coating, applicability to wide range of materials, easy control of the coating composition and suitability for automation.
US Patent Application No. 2014/0192500 is directed to a system, a packaged component and a method for making a packaged component, wherein the method comprises placing a component on a component carrier; encapsulating with an encapsulation body at least a portion of the component and the component carrier; and electrophoretic co-depositing organic molecules and inorganic elements thereby forming an insulating film on a conductive surface of the component, the component carrier or the encapsulation body.
There remains, however, an unmet need for an efficient, inexpensive, reproducible and easily automatable method for electrically insulating planar transformer conductive components, which would provide uniform coating encapsulating all the desired parts of the conductive component and being adequately adhered thereto.